1. Field of the Invention
The invention relates to salts of low molecular weight polymaleic acids and to their use as corrosion inhibitors in water-in-oil emulsions. The inhibitors are particularly useful in preventing the corrosion of metals caused by hydrogen sulfide and carbon dioxide in water-in-oil, particularly in saltwater-in-oil emulsions.
2. Description of the Prior Art
It is known to inject a solution or dispersion of corrosion inhibitors in crude oil during oil recovery and in the transport and storage of crude oil so that a protective layer forms on the surface of the metal parts coming in contact with the oil. The crude oil emulsions usually contain saltwater and, in many cases, depending upon the origin of the oil, hydrogen sulfide and carbon dioxide which have a pronounced corrosive effect. The corrosion inhibitors used for this purpose should be soluble in oil and should be at least dispersible in saltwater so that they can have an optimum effect.
Such systems are described in German Published application No. 28 46 977. These are imidazolinium salts which when used are dissolved in an oil-soluble organic solvent and in the presence of a hydrocarbon oil. The problem with such systems lies in the relatively complicated mode of metering three components and the fact that the corrosion protection is not satisfactory in all cases. We have determined, for instance, that the imidazoline having the formula ##STR1## described in the referenced publication (as chloride) does not develop sufficient effectiveness to counteract the corrosive erosion caused by the action of hydrogen sulfide.
Thus, an object of this invention was to find organic compounds which are optimally effective against the hydrogen sulfide and carbon dioxide corrosion in saltwater-in-oil emulsions and which are uniformly distributable in both phases without additional solvents.
Polymaleic acids in their free form or as simple salts are known as corrosion inhibitors from U.S. Pat. Nos. 4,105,581 and 4,018,702 as well as German published application No. 28 06 342. However, these substances only work in systems with pure water since they are not oil-soluble and require additions of phosphates or phosphonates (U.S. Pat. No. 4,105,581 and German published application No. 28 02 342), or zinc ions (U.S. Pat. No. 4,018,702) in order to bring about optimum results.
According to an earlier suggestion (U.S. patent application P No. 30 39 977), the oligomaleinates to be used in accordance with this invention are used as corrosion inhibitors in mineral oils, among other substances, also against hydrogen sulfide. In this case, the corrosion is, among other things, caused by hydrogen sulfide containing condensed water which does not form an emulsion with the oil. It is emphasized in this older application that the oligomaleinates are oil-soluble and do not have an emulsifying effect, that is, that they are not emulsifiable let alone soluble in the condensed water. It must, therefore, be all the more surprising that the materials can now be used in saltwater-in-oil emulsions, that is, that they are equally distributed in the water and the oil phase in such a manner that they can equally protect the corrosion endangered metal parts which are contacted by the oil as well as the water contrary to the problems described in Application P No. 30 39 977. This newly determined property thus facilitates the valuable expanded form of application of this invention.